1. Field of the Invention
The present invention relates to a fixing device employing an electromagnetic induction heating method used in an image forming apparatus such as a copier, a printer, a facsimile machine, a printing machine, and a multifunction machine combining several of the functions of these apparatuses, and to an image forming apparatus using the fixing device. More specifically, the present invention relates to a mechanism for setting a fixing area according to sheet size.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a toner image transferred to a recording medium, such as a recording sheet, from an image carrier, such as a photoconductor, is fixed on the recording medium by the action of fusion and penetration with heat and pressure, and thereby a copy is obtained.
Heating methods employed in the fixing device include an electromagnetic induction heating method. Unlike a commonly used heat roller fixing method, the electromagnetic induction heating method is advantageous in that the method does not require a heating mechanism, such as a heating roller, and is capable of generating heat by using an eddy current generated in a member used in the fixing process, such as a fixing roller or a belt, i.e., is capable of using the fixing member as a heating source and thus reducing the time taken to raise the temperature.
According to the electromagnetic induction heating method, however, it is difficult, in some cases, to equalize temperature distribution in the latitudinal direction of the fixing roller or the width direction of the belt owing to the relative thinness of an electromagnetic induction heat generating layer serving as a heat generating member. That is, if the recording sheet serving as the recording medium is fed in, for example, a width center alignment method in the latitudinal direction of the fixing roller or the width direction of the belt, heat loss and reduction in temperature occur in a sheet passing area corresponding to a central portion in the width direction of the recording sheet, while the heat loss and the resultant reduction in temperature are suppressed in sheet non-passing areas (i.e., areas of the fixing roller or belt outside the sheet, and over which the sheet does not pass) corresponding to opposite lateral end portions in the width direction.
In addition, recording sheets come in various standard sizes, such as A-series sizes and B-series sizes according to Japan Industrial Standards (JIS), for example. Further, in sheet feeding, the longitudinal direction parallel to the sheet feeding direction may be different between recording sheets of the same size. Due to the presence of the sheet passing area and the sheet non-passing areas, therefore, unevenness in temperature tends to occur in the latitudinal direction of the fixing roller or the width direction of the belt. Specifically, if a large-sized recording sheet is fed immediately after continuous feeding of small-sized recording sheets, the temperature distribution may be uneven in the width direction of the large-sized recording sheet and thereby adversely affect, for example, the glossiness of the image.
As a configuration addressing the unevenness in temperature in the latitudinal direction of the fixing roller or the width direction of the belt, a background heating mechanism includes a demagnetizing member which changes the influence of magnetic flux in the electromagnetic induction heating method. As well as an exciting coil which performs electromagnetic induction heating, a secondary demagnetizing coil is provided at each of positions corresponding to the sheet non-passing areas such that induced electromotive force and induced current of the secondary demagnetizing coil, which are generated by a change in magnetic flux generated by the exciting coil, reduce the magnetic flux in the sheet non-passing areas and thereby prevent an excessive increase in temperature.
Another background heating mechanism has a configuration including a metal plate and a magnetic shunt alloy having a characteristic of switching between magnetic and non-magnetic states at a Curie temperature. The magnetic shunt alloy is disposed between the metal plate and an exciting coil to allow a magnetic flux to pass through the magnetic shunt alloy into the metal plate when the temperature of the magnetic shunt alloy reaches or exceeds the Curie temperature. Thereby, a repulsive magnetic flux against the magnetic flux of the exciting coil is generated in the metal plate, cancelling out an induced magnetic flux generated by the exciting coil to provide a built-in temperature control capability.
Still another background heating mechanism has a configuration which includes magnetic flux cancellation coils facing the sheet non-passing areas, and which controls power supply to the magnetic flux cancellation coils to prevent an excessive increase in temperature in the sheet non-passing areas.
In the configurations of the background heating mechanisms, the following issue arises owing to a relatively wide thermal boundary between the sheet passing area and each of the sheet non-passing areas. Thermal boundary width refers to the space between an end portion of a heating area and an end portion of a non-heating area obtained in accordance with the overlapping position of the exciting coil and the demagnetizing cancellation coil at the boundary between the sheet passing area and the sheet non-passing area.
If the space between the end portion of the heating area and the end portion of the non-heating area is increased, the sheet non-passing area, which is not required to be heated, is also heated and increased in temperature. This results in an increase in energy loss, and may cause degradation of the fixing member due to the excessive increase in temperature.
Meanwhile, if the exciting coil and the demagnetizing cancellation coil excessively overlap each other between the end portion of the heating area and the end portion of the non-heating area, a portion of the sheet passing area adjacent to the sheet non-passing area is reduced in temperature when the demagnetizing cancellation coil is operated. As a result, unevenness in temperature occurs in the entire sheet passing area, and may cause phenomena such as a fixing failure and uneven glossiness of the image. Such phenomena occur in the configurations of the foregoing background heating mechanisms.